High-pressure metal-vapor electric discharge lamp



April 29, 1941. K. G. SCHNETZLER 2,240,353

HIGH'PRESSURE METAL-VAPOR ELECTRIC DISCHARGE LAMP Filed March 11, 1939 2 Sheets-Sheet l Fig.2.

50 I00 150 zoo 250 500 WAT T5 Inv entor: Karl C5. Schnetzler,

April 29, 1941.

HIGH-PRESSURE METAL-VAPOR ELECTRIC DISCHARGE LAMP Filed March 11, 1939 2 Sheets-Sheet 2 In vencor-:

Karl CiSchnebzler; by His Attorr ey.

K. e. SCHNETZLER 2, 40,353

Patented Apr. 29, 1941 UNITED STATES PATENT OFFICE HIGH-PRESSURE METAL-VAPOR ELECTRIC DISCHARGE LAMP Karl Gustav Sehnetzler,

E poration of New York asslgnor to General .Wembley Park, England, lectrio Company, a cor- Application March 11, 1939, Serial No. 261,390 In Great Britain March 11, 1938 .2 Claims. (01. 176-122) HPMV lamps in order to modify the color of the light from the discharge and especially to add red to it. Their presence has usually (so far as I am aware, always) reduced the luminous efiiclency. I have discovered that the addition of thallium, while not increasing the red content of the light, may increase the luminous efficiency considerably. Theincrease of efficiency appears to arise from the addition oi the thallium spectrum to the mercury spectrum; the efllciency with which the mercury spectrum is excited does not appear to be increased. The increase of eificiency is associated with a change of color, the light becoming greener. The U. V. efficiency may also be increased.

According to my invention the metal filling of a HPMV lamp of the type specified comprises thallium.

The vapor pressure of the thallium during operation may be adjusted so that the luminous efefliciency increases regularly with the pressure of the thallium vapor within a wide-range of that pressure. it is probable that it does not increase indefinitely; indeed my experiments indicate that I when the pressure attains a value (corresponding to 800 or 1000 C.) that is diflicult to attain in the present state of the art, the eiliciency may fall with further increase of pressure, for at least some values of the said relevant factors, especially when the pressure of the mercury is not much more than 1 atmosphere. Nevertheless, in the present state of the art, a safe general rule is that, if the highest possible efhciency is required,

' the pressure of the thallium should be made as great as is consistent with such other des'lderata as longlife, uniformity in manufacture, and staficiency of the discharge in full operation is substantially greater than that which would be obtained if the thallium were absent and the other relevant factors were unchanged. The relevant factors are the envelope, the electrodes and their position relative to each other and to the envelope, W the wattage dissipated, and VT the voltage between the electrodes in-full operation. In order that the increase in efficiency may be demonstrated, the value of VT can be restored for the same W when the thallium is subtracted, by modifying the ambient temperature or the quantity of mercury or both.

' Whatever the pressure of the mercury vapor (so long as it is always great enough to cause the lamp to be HPMV) thallium appears to have little effect on the efiiciency unless its pressure is at least as great as that corresponding to a temperature of 450 or even 500 C. (The pressure corresponding to a temperature '1 means, as usual, the pressure of vapor in equilibrium with thallium maintained at T".) Further, if the aforesaid relevant factors are, unchanged, the

bility in operation.

As is well known, the pressure of the thallium vapor can never exceed the limit corresponding to the temperature of the coolest spot on the boundary of the space to which the vapor has access. (If, as in known water-cooled lamps, the envelope has a narrow bore partly occupied by unevaporated metal, this coolest spot may be on the surface of the metal and not on the envelope itself.) Accordingly the said general rule means that the lamp should be designed so that the temperature of the said coolest spot is as great as the said desiderata permit, and that it is usually undesirable to limit the amount of thallium so that it is all evaporated at a dissipation much below that for which the lamp is designed, so that in normal operation the pressure is much below the said limit. Indeed it maybe desirable to have a considerable quantity of unevaporated thallium, in order that it may occupy irregularities'in the envelope, where the said coolest spot would otherwise lie; the said coolest spot will then, as indicated just above, lie on the surface of the thallium which will be slightly warmer than the envelope below it.

In respect of life, the determining considerations are much the same as those prevailing in normal HPMV lamps. Thus if the envelope is not more refractory than known quartz envelopes, and if the efficiency is not to decrease by more than 20% during the first hours of life, it is undesirable that W/S should watts/sq. cm., where S is the area of the internal surface of the envelope.

In respect of uniformity or manufacture and stability in operation it is sometimes preferable that the said coolest spot should not be behind the electrodes or otherwise at an irregularity of the envelope. For the temperature at such irexceed 40.

regularities is diflicult to control accidental differences in manufacture lead to considerable differences in the vapor pressure of the thallium, in the efliciency, and in operating characteristics. Then the temperature behind the electrodes may be maintained so high by known methods (such as placing the electrodes near to the end of the envelope, lagging, reflecting paints, and thickening of the wall of the envelope) that the coldest spot lies between the'electrodes and at a more regular and easily controllable place. On the other hand, it is not necessary to arrange that the coldest spot should lie on the substantially widest undistorted part of the envelope.

The vapor pressure of mercury at all relevant temperatures is much greater than that of thallium, It the pressure of the mercury vapor corresponded to thesame temperature as that of the thallium vapor, the total pressure and/or VT in the lamp would usually be undesirably high (except in water-cooled lamps). Accordingly it is usually desirable to limit the quantity of mercury present, in known manner, so that itspressure in full operation is much less than that corresponding to the said coolest spot. The procedure will therefore generally be to design the lamp, as aforesaid, with reference to the thallium pressure only and then to adjust the quantity of mercury so as to give the desired VT.

In selecting the quantity of mercury, it must be remembered that mercury and thallium form an amalgam over which the vapor pressure of mercury is less than over mercury-at the same temperature, and decreases with decreasing mercury content. If there is excess thallium, the mercury introduced will never be completely evaporated; the amount required to produce a given pressure will depend upon the amount or thallium-as well as on thevolume of the envelope.

These instructions concerning the quantity of mercury do not apply to water-cooled lamps of the known type aforesaid. In them it is usually necessary for known reasons that excess mercury should be present at the ends of the narrow tubes; a mercury pressure corresponding to a temperature nearly as high as that to which the thallium pressure corresponds may have to be tolerated.

The considerations that determine L, the length of the discharge column, are the same as inpure mercury lamps. The efiiciency generally increases with -W/L and V'r/L, so that a small L is desirable, subject to known limitations. It may be observed that, for a given W/L and Vr/L, the presence of thallium, while it increases efficiency, does not greatly increase the brightness of the discharge column.

One embodiment of the invention will now be described, by way of example, with reference to bore of the tube is 8 mm.; the electrodes are tungsten spirals enclosing a rod of alkaline earth silicate, and are 3 mm. long; the distance between them is 30 mm. The whole of the envelope behind the front edge of the electrodes and the seals 3 are coated with a reflecting layer '4. The envelope contains a portion of thallium 5 of which only a smal1 part is evaporated in operation, and also mercury 6 which is nearly all evaporated in full operation. The lamp is adapted to be operated with the straight line between the electrodes vertical and with the envelope surrounded by an evacuated jacket.

In the graph shown in Figure 2, it is assumed that, for each wattage dissipated, the quantity of mercury is adjusted so that VT is always volts. The abscissae are the watts dissipated; the ordinates the ratio of the light output when thallium is present t6 the light output when thallium is absent, W and VT being the same in both cases. The absolute efilciency in the presence of the thallium when W=250 is about 70 L/ watt.

In Figure 3 the'lamp'shown in Figure 1 is placed within a bulb I which is coated on its inner face with material 8, for example a mixture of zinc sulphide and zinc cadmium sulphide in equal proportions, adapted to be excited to luminescence by the radiation emerging through the envelope I. The bulb I is 150 mm. in diameter. 9, 9 are leads to the electrodes 2, 2. With this arrangement I have obtained a luminous efficiency of 55 lumens/watt at 250 watts.

I claim:

1. A high efiiciency discharge lamp having a particular power input rating and including spaced electrodes, an enclosure for the electrodes which is of such construction and proportions as to assure the attainment of an operating temperature of at least several hundred degrees centigrade when the lamp is operated at its said rated power input, a quantity of thallium within the said enclosure, and a quantity of mercury within the enclosure sufiicient to develop a vapor pressure at the said operating temperature in excess of the thallium pressure at such temperature but insufllcient to produce a condition of mercury vapor saturation, whereby both the thallium and mercury are enabled to generate light on an efllcient basis.

'2.- A high eiliciency discharge lamp having a particular, power input rating and including a pair of spaced electrodes, an enclosure for the electrodes which is of such construction and proportions as to assure the attainment of an operating temperature of at least four hundred fifty degrees centigrade when the lamp is operated I at its said rated power input, a quantity of thal- I lium within the enclosure sufiicient to produce a condition of thallium vapor saturation at the said operating temperature, and a quantity of mercury within the enclosure sufficient to develop a vapor pressure in excess of the thallium pressure at the operating temperature but in-,

sufficient to produce a condition of mercury vapor saturation, whereby both the thallium and mercury are enabled to generate light on an ef ficient basis, and the total efllciency of the lamp is in excess of that realizable with a lamp containing either mercury or thallium alone.

KARL GUSTAV SCHNETZLER. 

